Ebike BMS - Has anyone ever bridged the P- with the B- ?

[Crystaleyes]

Seeking a break from dodgy Chinese welders, I thought I'd get on with something else more straightforward... Boy was I wrong!

A friend gave me his e-bike battery and charger to have a look at.

It's a 36v 15a 10 cell system which has a 42v charger.

The charger is the type which shows a red LED whilst charging and a green when charged. The charger was displaying only green.
Measuring the battery voltage on the output read 36v. Only when any kind of load was connected, the voltage dropped to zero.
Opening the unit up, bypassing the BMS and connecting a 120v lamp to the batteries revealed that the batteries held a charge nicely, so I ordered a 'compatible' replacement BMS. Only problem being is that the Chinese replacement appears to be not working!

The BMS is the type which has three fat negative wires, B- (battery), C- (charging) and P- (power out / discharge) as well as 11 thinner wires connecting across each cell of the battery.

Partial schematic is below along with the new BMS board

Anyways, after installing the new board, the charger now displays the red LED as it charges the system. So far so good...
What it doesn't do, is have any electrical connection between the 'P-' output and the positive side of the battery bank.
That said, the B- terminal obviously does and gives a steady >36v across the batteries.

I know that the P- discharge pin is there to avoid overheating of the battery, but seeing as it isn' working, does anyone have any experience as to whether or not I am risking a lithium fire if I connect the P- power out wire directly to the B- on the BMS?
Another option could be to connect a current limiting resistor on the output -ve cable. 36v, 15 amps... Just need to confirm that the 15A is the load current and not the charge current, I suppose?

Or shall I just fuck it and see?

[Crystaleyes]

Re: Ebike BMS - Has anyone ever bridged the P- with the B- ?

So, if anyone is ever interested, or with a similar situation in the future...

I did connect the P- and B- wires, and now the battery charges nicely whilst discharging steadily using a lamp as a dummy load.

No current limiting resistor was added, as I think it is much more important to control the charging rate than the discharge rate, and not knowing the wattage of the motor, nor the Ah rating of the battery pack left me in doubt as to the best value and power rating of any resistor.

Gonna discharge and recharge the battery all day, before reassmbly and testing it on the ebike.

If it explodes or burns the house down, I'll let you know...

...

[sam_sam_sam]

Re: Ebike BMS - Has anyone ever bridged the P- with the B- ?

Ok this type of BMS protection is the type that you have use the one marked “P” the load the one that is marked “C” for charging

Now some BMS protection boards the “P” and “C” are electrically connected together and mostly marked “P”and sometimes marked “C” and sometimes both letters are used “P” and “C” together

Now if you are going to asked which type is better I can not answer you

However to me depending on your application this type has it advantages in that could the “C” on a solar panel and might not need an external diode but you would have to check and make sure that you can not power anything on the “C” terminal and that it is strictly for charging only

The other type has it advantages as well you have one less wire to deal with and easier to use at least for me

Now you should make sure that the BMS protection board does STOP charging when it reaches the voltage for the type of battery that it is made for I have seen some BMS protection board that I would hesitate to use because they would somewhat over charge the batteries or would discharge the batteries a little bit to low voltage that over time would shorten there

Also some BMS protection boards do not control amperage most of them do not but some do and the one that do are sometimes a pain to use because if you go over amperage this type you have to disconnect the the BMS protection board to reset it

I have tested all three types but the one thing that you need to carefully of is that under no circumstances you should ever have a battery become warmer than room temperature or not over 100*F while charging these types of batteries

Also note that Chinese made battery quality is questionable at best in my book this is not too say that ALL Chinese made batteries have quality issues but the problem is you do not know which ones have issues and which ones do not have quality issues unless you test them at least a few times the way you plan on using them

Japan made batteries are some of the best quality out there however there is an issue when the battery comes the end of it use full life they can become what is called “heater cells while charging “ this is dangerous to keep using them because can burst into flames and then have a real mess on your hands life ( if your battery charger does not have a temperature sensor system that actually works correctly )

This is why I highly recommend that if you are wanting to use used battery packs that you get a SkyRc battery tester at a minimum and that you set the temperature cutoff at 107*F which will turn off the charging cycle if you reach this temperature and it does work if setup correctly you do have to buy the temperature sensor separately it does not come with it and what ever you do do not buy a clone they are not the same and I personally would not trust them unless you know that you do NOT have have an issue with the battery packs that you are using

In this post I have bought a new device that I can set up for checking BMS / balancing/protection board but you have to make a testing jig to able to do this accurately which I have not put together yet I have to find some time for this project

Which means the following an adjustable switching power supply that has a voltage and current adjustment with a voltage and current meters one for each a good quality BMS balancing protection board the device I mentioned above

One note the type of switching power supply that has current and voltage that are adjustable and the meters voltage and current that are above 24 volts are a little pricey than just regular switching power supply just be aware of this

https://www.badcaps.net/forum/showthread.php?t=120607

[Crystaleyes]

Re: Ebike BMS - Has anyone ever bridged the P- with the B- ?

Thanks for that input. Don't really see myself buying one of those gadgets in the near future, but with all the ebike flying around, who knows?

As for the battery pack which I had here, the story was that this bike had been unused in a garage for some time due to the BMS having failed. I did try to find the exact same make and model, but seeing as it is quite old (2010?), then the best I could find were generic boards with the same ratings 36v 15A 10S.
The question arose, when there was no output from the battery when the -ve out wire was connected to the P- on the new board. Not sure if it was designed that way, or if the board was faulty?
I know that some of these boards can have the C- and P-combined, however this board also had no output circuit through the C- terminal, hence my connecting the P- wire to the B- terminal.

Yesterday, I gave the battery back and the bike happily whizzed around, up and down the local streets, putting on a couple of kilometers without seeming to lose any noticeable amount of charge (according to the LED charge gauge). As I said, I'd charged the battery pack several times and at no point did it heat up above even just slightly noticeable, so I'm not too concerned aout the 'charge' aspect of the bms. Any concern would be in the motor drawing the current too quickly, but as mentioned, I was unsure of exactly how best to resolve that?

Anyway, the owner knows that the new BMS isn't a straight replacement and to keep an eye on the discharging and charging over the next few cycles. I'll be seeing him on Friday so we should have a better idea by then...